Line level is the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. Generally, line-level signals sit in the middle of the hierarchy of signal levels in audio engineering.
Line level is the standard signal strength at which most audio equipment is designed to operate. Lower-level microphone and instrument signals must be brought up to line level with a preamp before interfacing with most equipment.
Simply stated, these both refer to the voltage level of an audio signal. Wired and wireless microphones are typically connected to mic-level inputs, whereas most other audio devices use line level. (We'll get to instrument- and speaker-level signals in a minute.)